Early human ancestors may have been using tools about 800,000 years earlier than thought, according to a new study based on newfound bone evidence—prehistoric leftovers linked to the famed "Lucy" fossil's species.

The discovery suggests, to at least one scientist, that tool use may extend as far back as five million years ago, to the last common ancestor of chimps and humans.

Found in East Africa, the two 3.4 million-year-old animal bones behind the new study appear to have been cut and crushed by stone tools wielded by the apelike human-ancestor species Australopithecus afarensis.

"You have all these images in museums and elsewhere showing Lucy walking through this East African landscape looking for food, and now we can put a stone tool in Lucy's hand," said study co-author Shannon McPherron, an archaeologist with the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

The finding, to be published tomorrow in the journal Nature, could force a scientific rethink of how the brain sizes of our early ancestors were affected by meaty diets.

A. Afarenisis Only Likely Candidate

Excavated from a dusty ridge in Ethiopia's Afar Basin, the two butchered bones include a rib (pictured above) from an unidentified cow-size animal and a thighbone from a goat-size antelope. Cut marks suggest that stone tools were used to remove the flesh from the bones and to extract marrow.

It's unlikely that the marks were been made by any other hominids, or hominins—members of our ancestral lineage and close evolutionary relatives—except A. afarensis.

"In this part of the world, at this time period, the only [hominid] species found to this point has been afarensis," McPherron said.

A. afarensis probably did not use their tools for hunting, he added. More likely, the early human ancestors were scavengers who used stones to butcher animal carcasses they came across.

Prior to the new discovery, the earliest direct proof of stone-tool creation and use among hominins dated to about 2.5 million years ago.

This younger evidence consists of marked bones and stone tools, which many paleoanthropologists think were left behind by Homo habilis, or "handy man," one of the earliest species of the human genus, Homo.

According to some scientists, the likely H. habilis tools are too well made to have been our evolutionary ancestors' first attempts at making tools.

"Scientists have been able to show that hominins [living around 2.5 to 2.6 million years ago] weren't just randomly going up to a cobble bed and selecting any kind of rock. ... ," said David Braun, a Paleolithic archaeologist at the University of Cape Town in South Africa.

"They were selecting certain types of rocks that were particularly good for making stone tools," said Braun, who was not involved in the new tool study but who wrote an accompanying commentary for Nature.

Because of the relative sophistication of the previous "oldest tools" record holders, he said, "many scientists have suggested there must be something older."

Now, apparently, there is.

But while the new findings suggest A. afarensis was using stone tools, there's no evidence the species was making them. It's possible that, like modern chimpanzees, Lucy and her ilk were using unaltered rocks.

The idea is that the "increased nutrients of meat allow you to grow a larger brain, which allows you to come up with novel solutions to make better stone tools, which allow you to get more meat," McPherron said.

The new tool findings are "a very important scientific discovery," said paleoanthropologist John Shea, who also was not involved in the study.

In addition to pushing back the advent of hominin tool use by almost one million years, the study opens up the possibility that human-ancestral tool use is even older—perhaps dating all the way back to when the ancestors of humans and chimpanzees split about five million years ago, said Shea, of Stony Brook University in New York State. (Related: "'Key' Human Ancestor Found: Fossils Link Apes, First Humans?")

"Humans and chimpanzees both habitually use tools, so it stands to reason that the last common ancestor was a tool user as well," he added.

Tool Use Passed Down Along Evolutionary Lines?

Another possibility raised by the new discovery is that tool use was a learned behavior, passed down among hominins, across different species and even genera—for example, from Australopithecus to Homo.

"We'll have to find more than these two bones, but if we fill in the record and we find more evidence of this, then we might be looking at a kind of learned behavior that was then shared and passed along in and amongst these groups," study co-author McPherron said.

An alternative explanation, Stony Brook's Shea said, is that different hominin groups discovered stone-tool use, and later stone-tool manufacture, independently.

"It's not that complex a behavior," Shea said. Tool use probably arose "again and again until it was locked in as a stable component amongst later hominins."